1. Field of the Invention
The present invention relates to a spark plug for mounting to an internal combustion engine for igniting an air-fuel mixture, and to a method of manufacturing the same.
The present invention can be applied to a device having a metal housing, such as a spark plug, a temperature sensor, and a gas sensor.
2. Description of the Related Art
Conventionally, an internal combustion engine uses a spark plug for ignition. An ordinary spark plug includes a center electrode whose front end portion serves as an electrode for generating spark discharge, an insulator which holds the center electrode in an axial hole thereof, and a metal housing which holds the insulator while surrounding the insulator from all radial directions. A mounting threaded portion which is formed on the outer circumferential surface of the metal housing is screwed into a mounting threaded hole in the internal combustion engine so as to mount the spark plug to the internal combustion engine. The thus-mounted spark plug generates spark discharge in the interior of a combustion chamber, thereby igniting an air-fuel mixture.
In recent years, implementation of improved output and low fuel consumption has been required of automobile engines. Under such circumstances, in order to ensure a degree of freedom in designing engines, there has been a demand for a reduction in the diameter of spark plugs. When, in order to meet the demand, the respective component sizes of a conventional spark plug are reduced in scale, the center electrode, the insulator, and the metal housing are reduced in diameter accordingly. However, reducing the diameter of the center electrode may lead to a reduction in thermal conductivity along the axial direction and deterioration in consumption resistance and thermal resistance. Thus, a reduction in the diameter of a spark plug is desirably implemented in conjunction with improvements in material strength and reduction in thickness of the metal housing.
Meanwhile, the metal housing has a seal portion which projects in the manner of a flange and is located rearward of the mounting threaded portion thereof. When the spark plug is mounted to an engine head of the internal combustion engine, a gasket intervenes between the seal portion and a peripheral portion around an opening of the mounting threaded hole in the engine head to maintain gastightness in the interior of the engine, the mounting threaded portion being screwed into the mounting threaded hole. A portion of the metal housing between the seal portion and the mounting threaded portion is a so-called screw neck and has a diameter smaller than the pitch diameter of the mounting threaded portion and the outside diameter of the seal portion. Furthermore, when a gasket is employed to airtightly seal a combustion chamber, in order to prevent detachment of the gasket, the screw neck has an engaging portion which is provided in the form of a groove along a circumferential direction of the metal housing and with which the gasket is engaged.
When the spark plug is fastened to the engine head, tensile stress is imparted to the screw neck. Particularly, tensile stress is apt to concentrate in a portion of the mounting threaded portion which is proximate to a threading start position at a rear end of thread formed on the mounting threaded portion. This portion is called a “thread rear-end portion.” More specifically, the thread rear-end portion ranges at least one pitch of thread as measured in axially opposite directions with the rear threading start position as the center of measurement. In the case where the thickness of the metal housing is reduced as mentioned above, if the spark plug is fastened to the engine head with a fastening torque greater than a predetermined value, cracking or rupture may occur in the thread rear-end portion.
A conceivable measure to cope with the above problem is to manufacture the metal housing from a steel material having a high carbon content so as to enhance its mechanical strength. However, this results in a loss of toughness. Therefore, when crimping is performed in the process of assembling the insulator with the metal housing, cracking may occur in a crimp portion of the metal housing. In order to cope with this problem, in the process of manufacturing the metal housing, a steel material is subjected to heating, finish rolling, and then cooling under predetermined conditions so as to impart, to the metal housing, a composite microstructure consisting of a ferrite phase accounting for a volume content of 60% or more and a phase formed through low-temperature transformation, which is martensite or bainite, or a mixed microstructure thereof. This can enhance both mechanical strength and toughness of the metal housing, so that the occurrence of cracking or rupture can be prevented particularly in the thread rear-end portion (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2-70019.